The superficial flexor muscle of the crayfish Procambarus clarkii is innervated by six axons which make connections with the 40-50 muscle fibers in a pattern that is correlated with the position of the individual muscle fibers on the surface of the muscle. When the nerve to this muscle is cut and left attached to a few muscle fibers, it regenerates within 5-10 weeks in a characteristic fashion, reestablishing connections comparable in specificity to the original connections. A muscle with its normal complement of synapses can also receive specific connections from a foreign nerve. We plan to continue the research of regeneration in this system in order to study at the cellular level the factors that could influence the regeneration of specific neuronal connections between two identifiable cell populations. We will use the presence of a connection, the mean size of junction potentials and the facilitation properties of the synapses to characterize and describe connection specificity. A variety of surgical procedures will be employed to manipulate the neuronal and muscle population to obtain nerve regeneration in a variety of contexts. We will change the number and position of the muscle fibers to analyze changes in connectivity that might result from alterations of the target area. We will selectively denervate the system and observe the regeneration process when some of the neurons are missing. We will introduce different foreign nerves and study the long term fate of foreign synapses in order to detect synaptic competition. We will repeat the regeneration experiments on very young animals in order to correlate changes in connectivity with the developmental stage of the animal. All the physiological observations of synaptic changes during regeneration in all these experiments will be correlated with anatomical changes by examining the synapses under the electron microscope. Analysis of the specificity of regeneration under these different conditions will give us insight into the mechanisms involved in the formation of specific neuronal connections.